Near infrared slitless spectroscopy with the Wide Field Camera 3, on board the Hubble Space Telescope, offers a unique opportunity to study low-mass galaxy populations at high redshift (z similar to 1-2). While most high-z surveys are biased toward massive galaxies, we are able to select sources via their emission lines that have very faint continua. We investigate the star formation rate (SFR)-stellar mass (M-star) relation for about 1000 emission line galaxies identified over a wide redshift range of 0.3 less than or similar to z less than or similar to 2.3. We use the Ha emission as an accurate SFR indicator and correct the broadband photometry for the strong nebular contribution to derive accurate stellar masses down to M-star similar to 10(7) M-circle dot. We focus here on a subsample of galaxies that show extremely strong emission lines (EELGs) with rest-frame equivalent widths ranging from 200 to 1500 A. This population consists of outliers to the normal SFR-M-star sequence with much higher specific SFRs (> 10 Gyr(-1)). While on-sequence galaxies follow continuous star formation processes, EELGs are thought to be caught during an extreme burst of star formation that can double their stellar mass in a period of less than 100 Myr. The contribution of the starburst population to the total star formation density appears to be larger than what has been reported for more massive galaxies in previous studies. In the complete mass range 8.2 < log(M-star/M-circle dot)< 10 and a SFR lower completeness limit of about 2 M-circle dot yr(-1) (10 M-circle dot yr-1) at z similar to 1 (z similar to 2), we find that starbursts having EWrest(H alpha) > 300, 200, and 100 angstrom contribute up to similar to 13%, 18%, and 34%, respectively, to the total SFR of emission-line-selected sample at z similar to 1-2. The comparison with samples of massive galaxies shows an increase in the contribution of starbursts toward lower masses.